From patchwork Mon Oct 17 17:06:27 2016
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Subject: [resend,
 4.9] hw_random: Don't use a stack buffer in add_early_randomness()
From: Andrew Lutomirski <luto@kernel.org>
X-Patchwork-Id: 9380037
Message-Id: <4169224b6858d1cf149f1a73f8a03603fa19076d.1476638125.git.luto@kernel.org>
To: linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org,
 Matt Mackall <mpm@selenic.com>, Herbert Xu <herbert@gondor.apana.org.au>,
 Rusty Russell <rusty@rustcorp.com.au>
Cc: Jens Axboe <axboe@fb.com>, Matt Mullins <mmullins@mmlx.us>,
 Andy Lutomirski <luto@kernel.org>
Date: Mon, 17 Oct 2016 10:06:27 -0700

hw_random carefully avoids using a stack buffer except in
add_early_randomness().  This causes a crash in virtio_rng if
CONFIG_VMAP_STACK=y.

Reported-by: Matt Mullins <mmullins@mmlx.us>
Tested-by: Matt Mullins <mmullins@mmlx.us>
Fixes: d3cc7996473a ("hwrng: fetch randomness only after device init")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---

This fixes a crash in 4.9-rc1.

resending because I typoed the git send-email command.  I stealthily added
Matt's Tested-by, too.

 drivers/char/hw_random/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 9203f2d130c0..340f96e44642 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -84,14 +84,14 @@ static size_t rng_buffer_size(void)
 
 static void add_early_randomness(struct hwrng *rng)
 {
-	unsigned char bytes[16];
 	int bytes_read;
+	size_t size = min_t(size_t, 16, rng_buffer_size());
 
 	mutex_lock(&reading_mutex);
-	bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
+	bytes_read = rng_get_data(rng, rng_buffer, size, 1);
 	mutex_unlock(&reading_mutex);
 	if (bytes_read > 0)
-		add_device_randomness(bytes, bytes_read);
+		add_device_randomness(rng_buffer, bytes_read);
 }
 
 static inline void cleanup_rng(struct kref *kref)